My main folders is the first level split of my photo categories. As the majority of the photos I take in a year are shot while on holiday, then that makes a natural 'first cut'. Within the category, Safaris have been separated out as these are primarily wildlife based. River trips, family holidays and non-wildlife trips come under 'Holidays'. Short city-breaks and other one off 'occasions' (such as our 4-day cooking course) come under Occasions and short trips.
There are two other main groupings: Airshows and Steam. On most years I make at least one trip to a major air-show; in July 2018 it will be to Duxford for their WW2 Flying Legends. For the shots in the Steam folder I normally make a trip to the Railway Museum at Didcot. However, this group also includes the odd diesel engine from the museum, as well as photos taken on a trip on the Flying Scotsman.
The folder for Other Genres is something of a catch-all; I wanted to include street photos and shots taken on 35mm film (there is a lot of overlap in these) as well as space for a fascination with shadows. Also, as I started with a bridge camera, and have now added a Lumix FZ1000 to my Fuji kit, I wanted a place for shots taken with this style of camera. Sometimes 'kit junkies' can be a bit sniffy about these cameras.